Irradiation of Samples for 40Ar/39Ar Dating Using the Geological Survey TRIGA Reactor GEOLOGICAL STRVEY PR O F E S Si I O N A L PAPER 1176 IRRADIATION OF SAMPLES FOR 40Ar /39Ar DATING USING THE GEOLOGICAL SURVEY TRIGA REACTOR Core of Geological SurveyTRIGA reactor in operation. Central thimble tube, which is about 4 cm in outside diameter and surrounded by control and transient rods, is in center of core. Blue glow is Cerenkov radiation, emitted by charged particles traveling faster than the speed of light in reactor cooling water. Irradiation of Samples for 40Ar/39Ar Dating Using the Geological SurveyTRIGA Reactor By G. BRENT DALRYMPLE, E. CALVIN ALEXANDER, JR., MARVIN A. LANPHERE, and G. PATRICK KRAKER GEOLOGICAL SURVEY PROFESSIONAL PAPER 1176 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Irradiation of samples for 40 Ar/ 39Ar dating using the Geological Survey TRIGA reactor. (Geological Survey professional paper ; 1176) Bibliography: p. 26-29 Supt. of Docs, no.: I 19.16:1176 1. Argon Isotopes. 2. Radioactive dating. 3. Nuclear reactors. I. Dalrymple, G. Brent. II. Series: United States. Geological Survey. Professional Paper 1176. QE508.I77 551.7'01 80-607859 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, B.C. 20402 CONTENTS Page Page Abstract ___ 1 Optimization of irradiation parameters Continued Introduction 1 Minimization of 40K(n, p)40Ar interference ______________ 26 The 40Ar/39Artechnique __________________________ 3 Minimization of 40Ca(n, no) 36Ar interference ___________ 26 The GSTR facility ___________________________________ 4 References cited ______________________________________ 26 Sample encapsulation __________________________________ 4 Derivation of equations _________________________________ 32 Monitor minerals ______________________________________ 8 40;'Ar/ Ar age equation _--__________---------_--- 32 Flux characteristics of the GSTR _________________________ 10 Error formulae ____________________________________ 33 Vertical gradients __________________________________ 12 37Ardecay correction _______________________________ 33 Horizontal gradients ________________________________ 14 Effect of 40Ca(n, no) 36Ar interference ___________________ 35 Self-shielding _____________________________________ 16 Computer programs ___________________________________ 42 Activity predictions ____________________________________ 16 NEUT2 Activity prediction program for GSTR ________ 42 Corrections for interfering argon isotopes __________________ 18 AR37 37Ar decay correction program _________________ 51 Optimization of irradiation parameters ____________________ 25 Selected bibliography __________________________________ 54 Production of sufficient 39Ar _________________________ 25 ILLUSTRATIONS Page FRONTISPIECE. Core of the Geological Survey TRIGA reactor in operation. FIGURE 1. Schematic cross section through U. S. Geological Survey TRIGA reactor ________________________________________ 5 2. Diagram showing details of GSTR core __________________________________________________________________ 6 3. Photograph showing partly assembled aluminum sample holder and quartz sample vials ___________________________ 7 4-8. Diagrams showing: 4. Cross section of a sealed 8-mm quartz sample vial with 10-mm high sample ______________________________ 7 5. Clamp for cooling quartz vials while they are being sealed ___________________________________________ 7 6. Aluminum sample holder _____________________________________________________________________ 8 7. Aluminum sample holder plates showing four useful arrangements of quartz vials of various sizes within an ir­ radiation level ___________________________________________________________________________ 9 8. Typical reactor tube for the central thimble ______________________________________________________ 9 9. Graph of approximate 39ArR productivity at centerline of central thimble as a function of K20 content and irradiation time 13 10. Graph of 40Arrad/39ArK ratio as a function of K-Ar age and irradiation time in central thimble of GSTR ________________ 14 11. Schematic diagram of change in true thermal power level as a function of fueling and calibration ____________________ 14 12. Graph showing vertical flux gradient in the central thimble of GSTR ___________________________________________ 14 13. Graph showing relative fluence in GSTR as measured with nickel wires and with the St. Severin monitor ______________ 15 14. Isometric drawing of sample locations in self-shielding experiment on diabase 8L691 ______________________________ 16 15-22. Graphs showing: 15. Results of self-shielding experiment on diabase 8L691 ______________________________________________ 16 16. Activity resulting from various neutron reactions in GSTR as a function of irradiation time atl MW ___________ 20 17. Effect of reaction 40Ca(n,na) 36Ar on calculated 40Ar/39Ar age ______________________________ 22 18. Approximate ranges of K/Ca atomic ratios for some common minerals and rocks ________________________ 22 19. Effectof 42 Ca(n,a) 39Ar reaction on calculated 40Ar/39Ar age ______________________________ 23 20. Effect of 40K(n,p) 40Ar and 41 K(n,d) 40Ar reactions on calculated 40Ar/39Ar age _________________________ 24 21. 39ArK/37ArCa as a function of K/Ca atomic ratio in GSTR ____________________________________ 25 22. Irradiation time in GSTR required to maximize the 39Ar produced from 39K while minimizing interferences from 40ArK and 36ArCa ____________-____--_____________________--_-_--___---__-_-_-__-__________ 25 23-35. Graphs showing effect of the reaction 40 Ca(n, no) 36Ar on calculated 40Ar/39Ar ages: 23. K/Ca = 0.02 ____________________________________________________-----_____________ 35 24. K/Ca=0.05 ____________________________________________________--__-__-__-__----_--------- 36 25. K/Ca =0.1 _______________________________________________-_______----____-__-_____ 36 26. K/Ca=0.2 ________________________________________________________---_-___-_-------------- 37 VI CONTENTS Page FIGURES 23-35. Graphs showing effect of the reaction 40Ca(n, no) 36Ar on calculated 40Ar/39Ar ages - Continued 27. K/Ca = 0.5 ________________________________________________________________________________ 37 28. K/Ca=l _______________________________________________________________________________ 38 29. K/Ca = 2 _________________________________________________________________________________ 38 30. K/Ca = 5 _______________________________________________________________________________ 39 31. K/Ca=10 _________________________________________________________________________________ 39 32. K/Ca=20 ________________________________________________________________________________ 40 33. K/Ca = 50 ______________________________________________________________________________ 40 34. K/Ca=100 ________________________________________________________________________________ 41 35. K/Ca = 200 _______________________________________________________________________________ 41 TABLES Page TABLE 1. Natural abundance and decay constants for selected isotopes of chlorine, argon, potassium, and calcium _____________________ 2 2. Analytical data for monitor minerals _____________________________________________________________________________________________ 10 3. Results for mineral standards irradiated in GSTR ________________________________________________________________________________ 11 4. Neutron energy distribution on the centerline of the central thimble of the GSTR ______________________________________________ n 5. Comparison of nuclear reactors used for 40Ar/39Ar irradiations ________________________________________________________________ 12 6. Data for activity calculations, TRIGA reactor _____________________________________________________________________________________ 17 7. Principal neutron reactions leading to the production of argon isotopes _________________________________________________________ 18 8. Correction factors for Ca- and K-derived Ar isotopes for the U. S. Geological Survey TRIGA reactor __________________________ 19 9. Standard mineral and rock compositions used inBASIC programNEUT2 ______________________________________________________ 45 10. Selected bibliography of use of Ar/ 9Ar data in terrestrial, lunar, and cosmological studies _________________________________ 54 IRRADIATION OF SAMPLES FOR <°Ar /39Ar DATING USING THE GEOLOGICAL SURVEY TRIGA REACTOR By G. BRENT DALRYMPLE, E. CALVIN ALEXANDER, JR.^MARVIN A. LANPHERE, and G. PATRICK KRAKER ABSTRACT ing has been investigated and utilized for geo- The characteristics of the Geological Survey TRIGA Reactor chronological studies by many laboratories throughout (GSTR) as a source of fast neutrons for the 40Ar/3 Ar technique of the world. Although most of the early studies involved K-Ar dating have been determined using data from more then 45 ir­ meteorites and lunar rocks, it soon became apparent radiations in the central thimble (core) facility. The GSTR has a flux over the entire energy spectrum of 1.1 x 1017 n/cm2 -MWH and a that the technique was a potentially powerful tool for fast/thermal ratio on the centerline of the central thimble of 117 for the investigation of terrestrial chronology. fast neutron energies greater than 0.6 MeV. Production of 39Ar is The U. S. Geological Survey's experiments on the about? x 10~13 mole/gram-percent KgO MWH, and